The present invention generally relates to digital information signal recording systems for recording digital information signals onto recording mediums, and more particularly to a recording system for recording digital audio information data which is obtained by subjecting audio signals to digital pulse modulation, onto a recording medium, by inverting a least significant bit (LSB) in one word of the digital audio information data when the bits in that one word are all "0" or all "1".
Generally, systems for recording an analog information signal onto a rotary recording medium (hereinafter simply referred to as a disc) as a digital information signal and reproducing the recorded signal as the original analog information signal, have been reduced to practice. When recording, the analog information signal such as an audio signal and a video signal is subjected to a digital pulse modulation such as a pulse code modulation (PCM), and formed into a digital signal. This digital signal is recorded onto the disc as variations in geometrical configuration constituted by rows of intermittent pits. When reproducing the recorded digital signal from the disc, variation in the intensity of light reflected from the disc or variations in electrostatic capacitance between the disc and a reproducing element is detected, to read and reproduce the recorded digital signal from the disc. Especially when the disc is of the electrostatic capacitance type from which the recorded digital signal is detected as variations in electrostatic capacitance between the disc and the reproducing element, the recorded digital signal can be reproduced within a wide band. That is, in the electrostatic capacitance type disc, it is possible to reserve a capacity for transmitting digital information data of 16 bits in four channels. Moreover, because it is possible to obtain a sufficiently wide transmission band according to the electrostatic capacitance type disc, it is advantageous in data transmission to carry out a frequency modulation which is hardly affected by dropout and level fluctuation.
As a data format for frequency-modulating the above digital signal and then recording the frequency modulated signal, the Non-Return-To-Zero (NRZ) format is advantageous in effectively using the band. However, the maximum number of continuous bits all containing "0" or all containing "1" may become large according to the information which is to be transmitted.
For example, one word from each of the 4-channel digital information data are added with a synchronizing signal, an error detecting code, and an error correcting code, to constitute a signal of one frame. Thus, the digital signal is a signal in which the signals in terms of one frame are time-sequentially multiplexed. Generally, a detection signal obtained by detecting edge portions of the digital signal reproduced from the disc, is used to form a clock signal which acts as a reference signal when reading the recorded data. However, if the data format of the digital signal is the NRZ format described before, a case where all or a majority of the bits in the 4-channel information data continuously assume the value "0" or continuously assume the value "1" occurs quite frequently. In such a case, edge portions do not exist in the digital signal. Thus, in such a case, it becomes difficult to obtain a clock pulse which is stably synchronized with the edge portions of the digital signal, and in extreme cases, the bits become out of synchronism.
It is possible to obtain a clock pulse which is stably synchronized with the edge portions of the digital signal when reproducing the digital signal from the disc, by first modulating the digital signal by a self-clocking modulation system (a bi-phase code modulation and the like, for example) and secondly frequency-modulating the modulated digital signal to record the frequency modulated digital signal. However, the digital signal which is modulated by the self-clocking modulation system has a frequency spectrum in which the energy is distributed largely in the high frequency range. On the other hand, the digital signal obtained from a frequency demodulator which demodulates the frequency modulated digital signal reproduced from the disc, has considerable noise in the high frequency range of its frequency spectrum where the energy is largely distributed. As is well known, this is because the noise spectrum of the frequency demodulation is such that the noise increases with respect to higher frequency components. This noise spectrum is the so-called triangle noise. Accordingly, there was a disadvantage in this method of recording the digital signal, in that there is considerable effect due to degradation in the signal-to-noise (S/N) ratio in the high frequency range. In addition, the hardware required for demodulating the digital signal which has been modulated by the self-clocking modulation system, was bulky and expensive compared to the hardware required with respect to the digital signal of the NRZ format described before.
Accordingly, the present assignee has proposed a system for recording the NRZ signal at random in a Japanese Patent Application No. 56-120359 (Japanese Laid-Open Patent Application No. 58-23310). According to this proposed digital information signal recording system, one block of signals is constituted by one word from each of the 4-channel digital information data, an 8-bit synchronizing signal having a fixed pattern, two kinds of error correcting codes, an error detecting code, and a 1-bit control signal for use upon random access and the like. The digital signal in this case, is a digital signal of the NRZ format in which signals in terms of one block are time-sequentially multiplexed. This digital signal is subjected to modulo-2 addition with a maximum length sequence generated from a generator polynomial x.sup.7 +x.sup.4 +1, for example. The digital signal obtained as a result of this modulo-2 addition is a randomized signal (this randomizing of the signal is called "scrambling"), and this scrambled signal is thereafter frequency-modulated and recorded onto the disc.
According to this proposed recording system, even when the bits in one word of the original information data are all "0" or all "1", the possibility of another value, that is, "1" or "0", existing within one word of the digital signal which is obtained by scrambling the data increases. In addition, the digital signal obtained by scrambling the data, has a frequency spectrum in which the energy is largely distributed in the low frequency range, as compared to the frequency spectrum of the digital signal which is obtained by the self-clocking modulation system. For this reason, compared to the case where the digital signal of he NRZ format is reproduced, it is possible to obtain a clock pulse which is more stably in synchronism with the edge portions of the digital signal when reproducing a disc recorded according to the above recording system proposed by the present assignee. Further, compared to the case where the digital signal modulated by the self-clocking modulation system is reproduced, the effect due to degradation in the S/N ratio in the high frequency range upon frequency demodulation can be reduced according to this proposed recording system. Moreover, it is possible to demodulate the digital signal of the NRZ format before it is scrambled, by use of an inexpensive circuit.
However, it has been confirmed that even when the data is scrambled by the maximum length sequence which is generated from the generator polynomial described before, there are cases where all of the bits in one word from each of the 4-channel information data within a signal corresponding to one block of the scrambled digital signal, continuously assume the value "0" or continuously assume the value "1". Hence, in such a case, the edge portions of the reproduced digital signal cannot be detected continuously for at least an interval corresponding to the transmission period of one word, and the clock information cannot be obtained. In order to obtain a clock pulse even in such a case, a tank circuit and a phase locked loop (PLL) coupled at a stage subsequent to the circuit which detects the edge portions of the reproduced digital signal, must have a considerably high accuracy. However, if such a tank circuit and PLL having high accuracy are employed, the cost of the reproducing apparatus became high.